Zio: Add Stream#mapAsync and Stream#mapAsyncUnordered

[iravid]

Signature:

trait Stream[R, E, A] {
  def mapAsync(n: Int)(f: A => ZIO[R1, E1, B]): Stream[R1, E1, B]
  def mapAsyncUnordered(n: Int)(f: A => ZIO[R1, E1, B]): Stream[R1, E1, B]
}

The first should execute up to n transformations in parallel and emit elements in the same order. The second is identical but can emit elements in the order of completion of f invocations.

[iravid]

Also, possible aliases: mapMPar, mapMParUnordered

[runtologist]

I'll give it a try. :)